Description of the payload

The instrument (also known as MASS-1 or MASS-89 in the specialized literature) was designed to measure electrons in the range of energies between 1.3 and 26 GeV and positrons from about 4 to 26 GeV. It was a modified version of a similar instrument flown in 1979 to observe antiprotons. The instrument design -as many others of those years- was an adaptation of the basic configuration of the so called Balloon-Borne Magnet Facility (BBMF), a payload developed by New Mexico State University and NASA's Goddard Space Flight Center to perform research on high energy particles using a magnet spectrometer.

A schematic diagram of the instrument used in this experiment is shown at left (click to enlarge).

It was composed by a superconducting magnet spectrometer, a time-of-flight scintillator system (TOF), a gas Cherenkov detector (G), a high resolution scintillator (S1), and a streamer tube imaging calorimeter.

The magnet spectrometer consisted of a single coil superconducting magnet that produced a maximum field strength of 2.2 Tesla, along with a tracking system that consisted of 8 planes of multiwire proportional chambers (MWPCs) read out by a delay line system. All eight of the MWPCs are equipped for measurement on the x-axis (maximum deflection) and four of the MWPCs have y-axis readouts. The MWPC are arranged with a pair of chambers at the top, middle and bottom of the spectrometer while there is a single chamber at the 1/4 height and another at the 3/4 height locations in the spectrometer.

The TOF system consisted of sixteen 1 cm thick paddles of plastic scintillator arranged in four planes (T1~T4) of four paddles each. T1 and T2 were located at the top of the instrument while T3 and T4 were located below the chambers of the magnetic spectrometer. A fifth high resolution scintillator (S1) was located on top of the upper plane of the tracking system. The coincidence among the four planes of the TOF system plus the pulse height of the S1 signal generated the trigger for an event to be recorded.

The Cherenkov gas detector system was located at the top of the gondola, in a 1 m tall cylinder isolated from the rest of the structure and filled with a mixture of Freon-12 and Freon-22. The Cherenkov light was reflected and focused by 4 segmented spherical mirrors onto four phototubes.

The imaging calorimeter was located below the bottom scintillator plane and permitted the reconstruction of the topological structure of the particle interactions. It consisted of 40 layers of brass streamer tubes with 64 readout channels per plane, stacked in alternating x-view and y-view layers.

Details of the balloon flight and scientific outcome

Launch site: Prince Albert Airport, Saskatchewan, Canada  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon SF3-424.37-080-NSCR-04
Balloon serial number: W29.47-3-07
Flight identification number: 273N

The balloon was launched by dynamic method using a crane as launch vehicle, on September 5, 1989.

After a initial ascent phase of near 3 hours, the balloon reached a float altitude of about 36 km and stayed there during 5 hours and 30 minutes.

After landing, the payload was retrieved from the launch site using a "Chinook" military helicopter.

In this mission was made for the first time an observation of cosmic-ray electrons and positrons using an imaging calorimeter, operated in conjunction with a magnetic spectrometer.

Although the flight was short, it has yielded data of excellent quality. Unambiguous identification of both positrons and electrons has been accomplished as well the detection of negative muons at different altitudes with good efficiency and low background providing the most extensive observation of this kind of the time.

External references and bibliographical sources

Images of the mission